Chapter on the Recommended Fasts of the Year
Man Lā Yaḥḍuruh al-Faqīh|Volume 2|Book 2|Chapter 4
وَ رُوِيَ أَنَّهُ سُئِلَ اَلْعَالِمُ عَلَيْهِ اَلسَّلاَمُ: عَنْ خَمِيسَيْنِ يَتَّفِقَانِ فِي آخِرِ اَلْعَشْرِ فَقَالَ «صُمِ اَلْأَوَّلَ فَلَعَلَّكَ لاَ تَلْحَقُ اَلثَّانِيَ ».
It is narrated that the Scholar, (Imam Musa ibn Jafar Al-Kadhim عليه السلام), was asked about two Thursdays that occur in the last ten days of the month. Imam عليه السلام said: "Fast the first one, for you may not live to see the second."
- Ṣaḥīḥ al-Faqīh